1. Field of the Invention
The present invention relates to a photomask and to an exposure method that uses that photomask. More particularly, the present invention relates to a photomask and to an exposure method that are suitable for fabricating large liquid crystal display devices.
2. Discussion of the Related Art
Generally, a liquid crystal display (LCD) device includes upper and lower substrates. The lower substrate includes switching devices and array lines, while the upper substrate includes a color filter. The upper and lower substrates are spaced apart from each other and a liquid crystal layer is interposed therebetween.
The lower substrate is often called an array substrate. It includes a plurality of pixel regions. Each pixel region is defined by crossing data lines and gate lines. A pixel electrode is disposed in the pixel region. At each data line/gate line crossing is a thin film transistor (TFT). Each TFT includes a gate electrode, a source electrode, a drain electrode, and an active layer. The gate and source electrodes electrically connect to the gate and data lines, respectively.
The above-mentioned elements are formed via a plurality of repeated production processes, including depositing, photolithography, and etching. Specifically, the photolithography process includes steps of: (1) coating a photoresist, (2) pre-heat treatment (pre-baking) of the photoresist, (3) exposure of the photoresist, (4) post exposure treatment (post baking), and (5) development. To form a desired photoresist pattern, a photomask is aligned with a photoresist-bearing substrate before exposure. Thereafter, desired portions of the photoresist layer are exposed to light.
A photomask aligning apparatus, for example, a scanner type Canon™ aligner model number MPA 5000, is conventionally used to align the photomask on the substrate. Only after each of the various photomasks is correctly aligned with the substrate is the desired photomask pattern successfully formed. Therefore, a plurality of first and second alignment keys are respectively formed on the photomask and on the substrate to precisely detect the relative position between the substrate and each of the photomasks. The first alignment keys, formed on the substrate, are called plate keys, while the second alignment keys in the photomask are called mask keys. Whether the photomask and the substrate are precisely aligned is determined by sensing the relationship between the mask keys and the plate keys.
A photomask typically includes a transparent substrate (beneficially quartz or glass) onto which either an emulsion thin film (an emulsion mask) or a chromium and chromium oxide thin film (a hard mask) are coated. In either event the photomask carries a desired photomask pattern, which includes the mask keys. The plate keys are formed on the substrate by a first photomask. Once the plate keys are formed, the other photomasks are aligned with respect to the plate key. Proper use of the alignment keys prevents misalignments between the substrate and the photomasks.
FIG. 1 is a plan view of a conventional photomask 17. As shown, a plurality of mask keys M1, M2, and M3 are formed on the photomask 17. The photomask 17 is designed in accord with the requirements of the Canon™ aligner. As shown, the photomask 17 has an overall width of 390 mm and a height of 610 mm (390×610). Additionally, the photomask 17 has a maximum exposure area of 330×560 mm2. However, a LCD device greater than 20-inches requires an exposure pattern greater than the 330×560 mm2 maximum of the conventional photomask 17. Therefore, the photomask 17 and the Caion™ aligner system are difficult to use when exposing an array substrate for a LCD device greater than 20-inches.
FIG. 2 illustrates the above-mentioned problem. For convenience, only first and second sub-areas 20a and 20b of a substrate 20 that is being patterned are shown. The substrate 20 is exposed to light, beneficially using a step-and-repeat exposure technology that is explained subsequently. After exposure, a non-exposed portion “A” results because of the limited exposure area (330 mm×560 mm) of the conventional photomask 17.
FIG. 3 illustrates a conventional step-and-repeat exposure technique using the conventional photomask. As shown, a large array substrate 31 is divided into a plurality of sub-areas. Each sub-area has plate keys P1, P2, and P3 along its side edges. Though not shown in FIG. 3, it should be understood that a photoresist layer is coated on the array substrate 31. As explained previously, the plate keys P1, P2, and P3 are formed using a first photomask (not shown). At first, a desired photomask (see FIG. 1) is aligned on a first sub-area “P” such that the mask keys (reference M1, M2, and M3 of FIG. 1) correspond to the plate keys P1, P2, and P3. At this point, first and second mask blades 33 and 35 cover the other sub-areas. When the alignment between the mask keys and the plate keys is correct, light is incident on the array substrate 31 such that only the first sub-area “P” is exposed to light through the desired photomask.
After the first sub-area P is fully exposed, the photomask and blades 33 and 35 are moved to a second sub-area. When the alignment between the mask keys and the plate keys is correct, the second sub-area is exposed to light through the photomask. All the sub-areas P are sequentially exposed using the same method. This process is the basis of the step-and-repeat exposure technology.
However, due to the problem explained in FIG. 2, the conventional step-and-repeat technology using the conventional Canon™ aligner system cannot be used for LCD devices greater than 20 inches.